Modify the size and location of the Rigid Body Collision Object
You can not resize the collision box itself, but you will notice that if you scale your object your collision box will scale as well.
To customize the collision box:


Scale your object in Edit Mode to fit the collision box.
Note the scale factors on paper and take the inverse of those numbers by calculating 1/x.
Then scale your Object by those Factors.


You can also do this with translation by translating in Edit Mode to fit the collision box and doing the inverse in Object Mode

Notice that this will work perfectly for cubes, but if you want spheres you have to keep them spheres. You can not turn spheres into ellipsoids.